Probe frequency response is usually measured by manufacturers under
ideal connections. In practical lab use, even short (1 cm and shorter)
leads connecting the probe to the circuit to be measured can have dramatic
effects on probe performance, especially for active probes that have a
capacitive input impedance. As discussed in the April,
2001 Technical Tidbit, the inductance of probe connections results
in an LC resonant circuit. The effects of such a resonant circuit can be
seen in Figure 1.

Figure 1 presents the results from a network analyzer measurement of
the input impedance of a popular 1.5 GHz active probe including the shortest
probe tip accessory furnished with the probe. The input impedance achieves
a minimum of less than 20 Ohms within the rated bandwidth of the probe!
Such an input impedance is likely to have a significant effect on the circuit
being measured. Both amplitude and timing errors may be introduced in the
measured circuit resulting in.problems than can be either caused or "solved"
upon connection of the probe to a circuit.

In addition to input impedance issues, the probe with its short connections
has a significant bump in its frequency response as seen in Figure 2. Note
the 7 dB gain peak at about 1 GHz. A signal at that frequency would appear
to have more than double its real amplitude. Other probe connection accessories
that come with many active probes are longer and make the gain peak much
larger and at at lower frequencies.

Figure 2. Frequency Response of the Probe to a 25 Ohm Source from Figure
1 with Short Connections

Most active probes are very sensitive to inductance in their connections
to the circuit. Even a few cm total for both leads is too much for most
probes. Many probe instruction manuals give the equivalent circuit for
the probe input impedance. Take the time to do a few calculations on this
circuit assuming 10 nH or so of inductance (short probing pins) in the
probe connections. You may be surprised at the results. And, just because
an accessory comes with a probe, do not assume it works. Many such accessories
designed to connect the probe to a circuit work poorly at best.

An example of a well written probe users manual for an active probe,
the Agilent 1158a, can be found at:

This users manual gives an accurate representation of probe's performance
including input impedance under a wide variety of conditions. It contains
good input impedance equivalent circuits and plots of impedance and frequency
response for the connection hardware the comes with the probe. It addition,
the probe itself contains design features I have been talking about for
many years in my seminars that give it a much improved frequency response
over most other active probes I have seen.

More information on this topic as well as design and troubleshooting
issues in electronic circuit design and manufacturing is covered in my
seminars. Click here for seminar
information.